1. Field of the Invention
The present invention relates to a read head with a combined second read gap and pinning layer for a spin valve sensor and, more particularly, to a second read gap layer that pins a magnetic moment of a pinned layer structure in the spin valve sensor.
2. Description of the Related Art
The heart of a computer is an assembly that is referred to as a magnetic disk drive. The magnetic disk drive includes a rotating magnetic disk, a slider that has read and write heads, a suspension arm above the rotating disk and an actuator that swings the suspension arm to place the read and write heads over selected circular tracks on the rotating disk. The suspension arm biases the slider into contact with the surface of the disk when the disk is not rotating but, when the disk rotates, air is swirled by the rotating disk adjacent an air bearing surface (ABS) of the slider causing the slider to ride on an air bearing a slight distance from the surface of the rotating disk. When the slider rides on the air bearing the write and read heads are employed for writing magnetic impressions to and reading magnetic signal fields from the rotating disk. The read and write heads are connected to processing circuitry that operates according to a computer program to implement the writing and reading functions.
An exemplary high performance read head employs a spin valve sensor for sensing the magnetic signal fields from the rotating magnetic disk. The sensor includes a nonmagnetic electrically conductive spacer layer sandwiched between a ferromagnetic pinning layer and a ferromagnetic free layer. An antiferromagnetic pinning layer interfaces the pinned layer for pinning the magnetic moment of the pinned layer 90xc2x0 to an air bearing surface (ABS) wherein the ABS is an exposed surface of the sensor that faces the rotating disk. First and second leads are connected to the spin valve sensor for conducting a sense current therethrough. A magnetic moment of the free layer is free to rotate upwardly and downwardly with respect to the ABS from a quiescent or zero bias point position in response to positive and negative magnetic signal fields from the rotating magnetic disk. The quiescent position of the magnetic moment of the free layer, which is preferably parallel to the ABS, is when the sense current is conducted through the sensor without magnetic field signals from the rotating magnetic disk. If the quiescent position of the magnetic moment is not parallel to the ABS the positive and negative responses of the free layer will not be equal which results in read signal asymmetry which is discussed in more detail hereinbelow.
The thickness of the spacer layer is chosen so that shunting of the sense current and a magnetic coupling between the free and pinned layers are minimized. This thickness is typically less than the mean free path of electrons conducted through the sensor. With this arrangement, a portion of the conduction electrons is scattered by the interfaces of the spacer layer with the pinned and free layers. When the magnetic moments of the pinned and free layers are parallel with respect to one another scattering is minimal and when their magnetic moments are antiparallel scattering is maximized. An increase in scattering of conduction electrons increases the resistance of the spin valve sensor and a decrease in scattering of the conduction electrons decreases the resistance of the spin valve sensor. Changes in resistance of the spin valve sensor is a function of cos xcex8, where xcex8 is the angle between the magnetic moments of the pinned and free layers. When a sense current is conducted through the spin valve sensor, resistance changes cause potential changes that are detected and processed as playback signals from the rotating magnetic disk. The sensitivity of the spin valve sensor is quantified as magnetoresistance or magnetoresistive coefficient dr/R where dr is the change in resistance of the spin valve sensor from minimum resistance (magnetic moments of free and pinned layers parallel) to maximum resistance (magnetic moments of the free and pinned layers antiparallel) and R is the resistance of the spin valve sensor at minimum resistance. The transfer curve for a spin valve sensor is defined by the aforementioned cos xcex8 where xcex8 is the angle between the directions of the magnetic moments of the free and pinned layers. Because of the high magnetoresistance of a spin valve sensor it is sometimes referred to as a giant magnetoresistive (GMR) sensor.
An improved spin valve sensor, which is referred to hereinafter as antiparallel pinned (AP) spin valve sensor, is described in commonly assigned U.S. Pat. No. 5,465,185 to Heim and Parkin which is incorporated by reference herein. The AP pinned spin valve differs from a single pinned layer spin valve sensor in that the pinned layer comprises multiple layers, hereinafter referred to as an AP pinned layer structure. The AP pinned layer structure has a nonmagnetic spacer layer which is sandwiched between ferromagnetic first and second AP pinned layers. The first AP pinned layer, which may comprise several thin films, is immediately adjacent to the antiferromagnetic pinning layer and is exchange-coupled thereto, with its magnetic moment directed in a first direction. The second AP pinned layer is immediately adjacent to the spacer layer and is exchange-coupled to the first AP pinned layer by the minimal thickness (in the order of 6 xc3x85) of the spacer layer between the first and second AP pinned layers. The magnetic moment of the second AP pinned layer is oriented in a second direction that is antiparallel to the direction of the magnetic moment of the first AP pinned layer. The magnetic moments of the first and second AP pinned layers subtractively combine to provide a net moment of the AP pinned layer structure. The direction of the net moment is determined by the thicker of the first and second AP pinned layers. The thicknesses of the first and second AP pinned layers are chosen so that the net moment is small. A small net moment equates to a small demagnetizing (demag) field exerted on the free layer by the AP pinned layer structure. Since the antiferromagnetic exchange coupling is inversely proportional to the net moment, this results in a large exchange coupling between the pinning and AP pinned layers.
A spin valve sensor can be classified as either a top spin valve sensor or a bottom spin valve sensor. In a top spin valve sensor the pinned layer structure is located closer to the second read gap layer than to the first read gap layer, and in a bottom spin valve sensor the pinned layer structure is located closer to the first read gap layer than to the second read gap layer. A spin valve sensor can be further classified as a single spin valve sensor or a dual spin valve sensor. In a dual spin valve sensor the free layer structure is located between first and second copper (Cu) spacer layers, the first and second spacer layers are located between first and second pinned layer structures and the first and second pinned layer structures are pinned by first and second pinning layers. As indicated hereinabove, a spin valve sensor can be further classified as a single pinned layer spin valve sensor or an AP pinned spin valve sensor.
The storage capacity of a computer depends upon the areal density of each of the read and write heads. The areal density of the read head is a product of its linear read density and its track width density. The track width density is quantified as the number of tracks per inch along the radius of a rotating magnetic disk. The linear density of the read head is quantified as the number of bits that can be read by the read head per inch of a track width along a circular track on the magnetic disk. The linear density of the read head depends upon the total read gap which is measured between the first and second shield layers of the read head. Accordingly, in order to increase the linear density of the read head the thicknesses of the layers of the spin valve sensor and the first and second read gap layers should be minimized. A major contributor to low linear density is the thicknesses of the first and second read gap layers. The first and second read gap layers, which are typically aluminum oxide (Al2O3), must be thick enough to prevent electrical shorting between the spin valve sensor to the first shield layer and between the first and second leads to the second shield layer. When either of these read layers become too thin there is a risk of pin holes which will permit current to short between the sensor and/or the leads to the shield layers. There is less risk of shorting between the sensor and the first shield layer since these layers are planarized. However, the first and second lead layers have steps which must be covered by the second read gap layer. The highest risk of shorting is where the second read gap layer covers the steps of the first and second lead layers since there is typically a thinning of the second read gap layer at the step locations. Accordingly, the second read gap layer is typically thicker than the first read gap layer in order to provide for adequate coverage of the steps of the first and second lead layers.
In order to increase the storage capacity of a computer there is a strong-felt need to decrease the thicknesses of the first and second read gap layers. As discussed hereinabove, however, this is restricted by the risk of shorts between the sensor and the lead layers to the shield layers.
The present invention provides a read head with a combined second read gap and pinning layer for pinning a pinned layer structure of a top spin valve sensor so as to significantly increase the linear read density of the read head. This is accomplished by providing a second read gap layer that is at least partially composed of alpha ferric oxide (xcex1Fe2O3). The alpha ferric oxide (xcex1Fe2O3) is an insulative material which serves as a second read gap between the spin valve sensor and the second shield layer. Further, the alpha ferric oxide (xcex1Fe2O3) pins the pinned layer structure by providing the pinned layer structure with high coercivity. Accordingly, the net magnetic moment of the pinned layer structure can be pinned perpendicular to the ABS in a direction either into the sensor or out of the sensor as desired. An applied magnetic field is employed for setting the magnetic moment in the desired direction.
In one embodiment of the invention the second read gap layer may be made entirely of alpha ferric oxide (xcex1Fe2O3) and in a second embodiment of the invention the second read gap layer may be made of first and second films wherein the first film is composed of alpha ferric oxide (xcex1Fe2O3) and interfaces the pinned layer structure. In the second embodiment the second film can be also composed of alpha ferric oxide (xcex1Fe2O3) or of aluminum oxide (Al2O3). With the present invention the second read gap layer can be made thick enough to cover the aforementioned steps caused by the first and second lead layers without unduly reducing the linear read density of the read head. With the present invention a separate antiferromagnetic (AFM) pinning layer is not required since the second read gap layer performs that function. By eliminating the separate pinning layer, which can be several hundred angstroms thick, the total read gap between the first and second shield layers is reduced by that amount.
In a preferred embodiment longitudinal biasing of the free layer structure in order to stabilize its magnetic domains is accomplished by first and second antiferromagnetic layers located in first and second passive regions of the read head, respectively, in lieu of first and second hard bias layers which typically abut first and second side edges of the sensor. With this arrangement the magnetic spins of the first and second antiferromagnetic biasing layers may be set by a longitudinal applied field in the presence of a high temperature such as 375xc2x0 C. for platinum manganese (PtMn). Subsequently, the coercivity of the pinned layer structure can be uniaxially set perpendicular to the ABS by applying a high magnetic field without the presence of heat. Since heat is not required in the setting of the pinned layer structure the magnetic spins of the first and second antiferromagnetic biasing layers will not be altered.
An unexpected advantage of the present invention is that the alpha ferric oxide (xcex1Fe2O3) of the second read gap layer serves as a specular reflector for reflecting conduction electrons back into the mean free path of conduction electrons between the free layer structure and the pinned layer structure. Accordingly, as conduction electrons attempt to escape the mean free path they are reflected back to the mean free path for improving the magnetoresistive coefficient dr/R of the read head.
An object of the present invention is to provide a read head with a second read gap layer which pins a pinned layer structure of a top spin valve sensor.
A further object is to provide a read head with a combined second read gap and pinning layer for a top spin valve sensor which permits a significant increase in the linear read density of the read head.
A still further object is to provide a read head with a combined second read gap and pinning layer which increases the linear read density of the read head and reflects conduction electrons in a spin valve sensor for improving the magnetoresistive coefficient dr/R.
Other objects and advantages of the invention will become apparent upon reading the following description taken together with the accompanying drawings.